Interaction of aloe active compounds with calf thymus DNA

Natural anthraquinone compounds have emerged as potent anticancer chemotherapeutic agents because of their promising DNA-binding properties. Aloe vera is among one of the very well-known medicinal plants, and the anthraquinone derivatives like aloe emodin (ALM), aloins (ALN), and aloe emodin-8-glucoside (ALMG) are known to have immense biological activities. Here, we have used biophysical methods to elucidate the comparative DNA-binding abilities of these three molecules. Steady-state fluorescence study indicated complexation between calf thymus DNA (ctDNA) and both the molecules ALM and ALMG whereas ALN showed very weak interaction with DNA. Displacement assays with ctDNA-bound intercalator (ethidium bromide) and a groove binder (Hoechst 33258) indicated preferential binding of both ALM and ALMG to minor groove of DNA. Isothermal titration calorimetric (ITC) data suggested spontaneous exothermic single binding mode of both the molecules: ALM and ALMG. Entropy is the most important factor which contributed to the standard molar Gibbs energy associated with relatively small favorable enthalpic contribution. The equilibrium constants of binding to ctDNA were (6.02 ± 0.10) × 10⁴  M^-1 and (4.90 ± 0.11) × 10⁴  M^-1 at 298.15 K, for ALM and ALMG, respectively. The enthalpy vs temperature plot yielded negative standard molar heat capacity value, and a strong negative correlation between enthalpy and entropy terms was observed which indicates the enthalpy entropy compensation behavior in both systems. All these thermodynamic phenomena indicate that hydrophobic force is the key factor which is involved in the binding process. Moreover, the enhancement of thermal stability of DNA helix by ALM and ALMG fully agreed to the complexation of these molecules with DNA.